Comparative Physiology of the Brain and Comparative Psychology

3. In considering the brain of Vertebrates we are obliged to deal with the brain of the cold-blooded animals, for the simple reason that in warm-blooded animals we cannot well perform brain-operations in the vicinity of the medulla oblongata without having the respiration cease. In cold-blooded animals the shock-effects are not so great. We have selected the brain of the frog as a type because it has been worked out the most carefully. It consists chiefly of the cerebral hemispheres (GH, Fig. 35), thalamus op- ticus (T/i., O), optic lobe, cerebellum (AT/), and medulla oblongata. The diagram (Fig. 35) gives the origin of the nerves of the brain (V-XI). It is our aim to show in this chapter that the individual activi- ties of the frog are dependent upon the segmental ganglia and that we have no right to speak of " cen- tres " for the single activities unless the word centre is synonymous with the expression segmental ganglion.

movements. It was for a long time a dogma that progressive locomotive movements could only be per- formed by frogs that were still in possession of their cerebral hemispheres. This statement was made by Flourens. He observed that frogs devoid of the cerebral hemispheres no longer move spontaneously (3). Later on Schra- der showed that this observation was not correct ; that this lack of spontaneity only occurs when the thai- ami optici are injured (4). Are we to con- clude from this that the power of spontan- eous locomotion is lo- cated in the thalami optici ? This would be wrong, for if the whole brain of a frog including the pars commissuralis of the medulla oblongata be removed, it seems " possessed of an irre- sistible desire to move ; it creeps about untiringly in an entirely coordinated manner and does not rest until it comes to a corner of the enclosure " (Schrader). It behaves like the Nereis in Maxwell's experiments which was deprived of its brain. Flourens made his

Gff, cerebral hemispheres; Tk.O, thalamus opticus; Lob. oft, lobi optici ; A'//, cerebellum ; l^-JCf, origin of the sth to nth brain-nerves. (After Wiedersheim.) localisation too high up. We wish to emphasise the fact that the frogs from which Schrader removed the whole brain, including the pars commissuralis, not only moved but were still able to climb. The con- dition of rest which appears after injury to the thala- mus is thus not due to the loss of spontaneity. Steiner has also attempted to localise locomotion in a " cen- tre." He found that frogs after losing the pars com- missuralis of the medulla made no more progressive movements, and concluded from this that the sole and undivided control overall locomotions of the body belongs to this part (5 and 6). Schrader's contradic- tory results overthrow Steiner's conclusions. The latter author evidently made his observations on mor- ibund animals, for his frogs survived the operation only a week at the most, while Schrader's lived many months and entirely recovered from the operation.

According to the segmental theory, on the other hand, it is to be expected that only those parts of the central nervous system are necessary for locomotion, which correspond to the segments of the muscles of the arms and legs. Thus it must be possible to ob- tain coordinated locomotion as long as the segmental ganglia of the muscles of the arms and legs are intact. This agrees with the result obtained by Schrader that after extirpating the whole brain, including the pars commissuralis, coordinated locomotion still oc- curs. We can go still farther and extirpate the whole medulla as far as the tip of the calamus scriptorius and still obtain coordinated locomotion. " Disturb-

ance of the coordination in movements first begins with the apparent decrease in the ability to use the fore- legs, which becomes more and more apparent the nearer the incision approaches the origin of the bra- chial plexus from the tip of the calamus scriptorius. When this is reached the animal falls flat on its belly ; the fore-legs are no longer able to carry the body. If the animal be stimulated in the middle-line, for in- stance at the anus, the hind-legs throw the body for- ward. The forward extremities participate still with 4 alternating ' but insufficient and peculiar trembling movements. A really coordinated progressive move- ment no longer takes place." We see again in this case the entire validity of the segmental theory : in- juries of the spinal cord in the vicinity of the brachial plexus interfere with the walking movements only in so far as the cooperation of the fore-legs comes into consideration. The hind-legs, on the other hand, continue to function normally. Similar phenomena may be observed in fishes. They also cease to move about when the brain is removed up as far as the medulla oblongata. It would be quite wrong, how- ever, to conclude from this that the centre of locomo- tion is located in the medulla oblongata. If the head of a shark be amputated the body swims about spon- taneously This experiment was made by Steiner. From the standpoint of the segmental theory this re- sult was to be expected. The tail is the organ of locomotion for the shark, and only the corresponding segmental ganglia of the spinal cord are required for

its activity. If the spinal cord of a young salamander be severed the swimming movements of the anterior and posterior parts are so well coordinated that it hardly seems credible that an operation has been per- formed. The same is true of the eel (8). The con- ditions are about the same as in the earthworm. Rubbing the back of the frog causes it to croak, and in a frog whose brain has been removed as far as the medulla oblonerata. this sound, as Goltz has

found, can be produced with machine-like regularity (2). Viewed from the segmental standpoint this reflex is naturally conditioned by the integrity of the me- dulla, since it is there that the motor nerves for the production of the voice originate. The centre-theory had found a supposed " centre " for this reflex higher up in the brain. 4. The instinct for food and self-preservation was, like all the instincts, located in the cerebral hemi- spheres. An analysis of this instinct shows that it is composed of several reflexes, which are discharged successively. The first is a visual reflex ; the frog catches only objects (flies, for instance) that are in motion. The opticus ends in the thalamus opticus, hence it is to be expected that the loss of the cerebral hemispheres would not prevent the frog from catch- ing flies. Schrader found this to be the case. If previous authors believed their experiments to prove that the cerebral hemispheres are necessary for see- ing, they were misled by the shock-effects of the operation, and in this way made the localisation too

high. Goltz had already shown, moreover, that the frog deprived of its cerebral hemispheres avoids ob- stacles. The same holds good for the fish that has lost the cerebral hemispheres. The first act in taking food thus consists in an optical reflex. As soon as the food comes in contact with the palate it arouses swallowing reflexes. These reflexes are completed by means of the vagus group. According to the segmental theory these reflexes should still be possi- ble even when all the parts of the brain lying in front of the nuclei of the vagus have been removed. Such is the case. As long as the medulla oblongata is preserved the frog swallows the food that is put into its mouth.

The respiration of frogs is chiefly mouth- and neck- respiration. The corresponding nervous segments for these parts of the body lie in the medulla oblon- gata and in the beginning of the spinal cord. If the latter be severed behind the noeud vital (calamus scriptorius), as Schrader found, all the muscles whose nerves originate behind the place of division continue to participate in the respiration coordinately. It was formerly assumed that the compensatory movements of frogs were dependent on organs of the mid-brain. Schrader found, however, that frogs whose brain had been extirpated as far as the medulla oblongata (the origin of the acusticus) still showed compensatory movements. The earlier physiologists were deceived by accidental effects of the operation. For the sake of completeness it should be mentioned,

further, that the reflexes of wiping and warding off objects from the body of the frog are of a purely segmental character. We have now given a review of the principal reac- tions of the frog and have found that no localisation of functions exists either in the brain or the spinal cord, that these are only segmental reflexes, just as in the Annelids and Arthropods. This conception was natural after results obtained from experiments on lower animals. That Schrader had foreseen it before the experiments reported here were known is proved by the closing sentence of his article on the frog's brain : " The series of experiments we have given teaches us that the central nervous system of the frog can be divided into a series of sections, each of which is capable of performing an independent function. It brings the central nervous system of the frog into closer relation with the central nervous system of the lower forms, which consists of a series of distinct gan- glia that are connected by commissures. It speaks against the absolute monarchy of a single central ap- paratus and against the existence of different kinds of centres, and invites us to seek for the centralisation in a many-sided coupling of relatively independent sta- tions." The question might be raised as to whether the activities of the frog which we have considered include all the reactions of this animal. The more complicated instincts are for the most part nothing more than series of segmental reflexes. I am inclined to recommend using the word chain-reflexes, whereby

the performance of one reflex acts at the same time as the stimulus for setting free a second reflex. The tak- ing of food may serve as an illustration of such a chain- reflex. The optic reflex of the moving fly produces the snapping reflex ; the contact of the mouth-epi- thelium with the fly produces the swallowing reflex. Each of these reflexes is purely segmental. By tak- ing into account the act of transmission, complicated acts can thus be resolved into a few segmental reflexes. A second fact must be taken into consider- ation if we wish to trace back the reactions of a frog to segmental reflexes, namely, that the irritability of the organs of its body changes. In the chapter on instincts we shall find how chemical conditions, espe- cially, affect the form of the irritability of the animal, and how all conditions which bring about chemical changes in the body (temperature, food, sexual pro- ducts) also modify its irritability. We shall then un- derstand why the frog burrows at the beginning of the cold weather in autumn and puts in an appearance again with the awakening of spring, or, strictly speak- ing, with the beginning of the warm weather. The segmental reflex in the frog is, however, determined also by the irritability of the peripheral organs and the arrangement of the muscles. The segmental ganglion acts, in the main, simply as the protoplasmic connec- tion between the surface of the body and the muscles. The experiments of Goltz and of Goltz together with Ewald on the spinal cord of dogs prove that this law of segmental reflexes is also correct for dogs.

However, in warm-blooded animals every operation in the vicinity of the medulla oblongata is accompanied by such severe shocks to the segmental respiratory ganglia that the experimental proof is still wanting for the ganglia of the medulla in higher Vertebrates. It has been attempted with electrical stimuli, but such experiments only show that some kind of proto- plasmic connection exists between the stimulated spot and the segmental ganglia of the active muscles. The fact, for instance, that the respiratory movements are affected by stimulation of the third ventricle only proves that there are fibres at that place which go to the segmental respiratory ganglia. The conclusion, however, cannot legitimately be drawn from this that respiratory ganglia or " respiratory centres" are located in the third ventricle. Two facts have combined to hinder the development of the segmental theory. First, comparative physiology of the brain and embry- ology have never been duly considered. Because the brain of Vertebrates only reveals its segmental char- acter in the earliest embryological condition, only a small number of physiologists have thus far seriously believed that the segmental character of the central nervous system would furnish the key for comprehend- ing its functions. The second fact is disregard of the shock-effects upon those parts of the central nervous system situated behind the seat of the operation. It is possible that certain impulses flow constantly from the cephalic to the lower parts of the central nervous system. The stopping of these influences

ganglia behind the seat of the operation. This change is the shock-effect. Finally, the most important difference between the segmental conception of the central nervous system and the centre-theory may be pointed out. Accord- ing to the latter theory, the central nervous system consists of a series of centres for as many different "functions." Each "function" is determined by the structure of its "centre." According to the seg- mental theory, there are only indifferent segmental ganglia in the central nervous system, and the dif- ferent reactions or reflexes are due to the different peripheral organs and the arrangement of muscles. The centre-theory must remain satisfied with the mere problem of localising the apparent " seat " of a " function " without being able to give the dynamics of the reactions of an animal, as the latter depend in reality upon the peripheral structures, and not on the structures of the ganglia. For this reason the segmental theory alone will be able to lead to a dynamical conception of the functions of the central nervous system.

This difference may be made more apparent by comparison of these functions with those of the re- tina. The optical perception of forms consists in the power of single elements to determine, accord- ing to their position on the retina, different space- sensations. One retinal element may aid in bringing about many different pictures. Viewed from the segmental standpoint, we imagine the role of the central nervous system to be similar to this : the various elements or ganglia take the place of the re- tinal elements in the perception of forms. The same elements or ganglia participate in many " functions." Every element shares in the result according to the location of the segment, and other general or special qualities. But if we attempt to make clear to our- selves how the retina should act according to the centre-theory, we find that every retinal element would have to serve for the perception of one image only, that we could see only as many different images as we have retinal elements (for instance, rods). We do the centre-theory no injustice in making this com- parison : its consistent representatives really assume that each image of memory is deposited in a special cell, that the number of the cells of the brain de- termines the number of the images of memory which are possible.

I wish to call the attention of the reader to the fact that Dr. A. Meyer has arrived, independently, at simi- lar conclusions concerning the segmental character of the central nervous system of Vertebrates as those set forth in this chapter (9). 1. GOLTZ. Ueber die Functionen des Lendenmarks des Hundes. Pfliigers Archiv, Bd. viii., 1874. 2. GOLTZ. Beitrdge zur Lehre von den Nervencentren de& Frosches. Berlin, 1868. Verlag von Hirschwald. 3. FLOURENS, P. Richer dies experimentalcs sur les Proprie'te's et les Fonctions du Systems Nerveux, etc. 2. edit. Paris, 1842.

4. SCHRADER, MAX E. G. Zur Physiologic des Froschgehirns. Pfliigers Archiv, Bd. xli., 1887. 5. STEIN ER, J. Die Functionen des Centralncrvcnsystems und ihre Phylogcnese. Erste Abtheilung : Untersuchungen uber die Physiologic des Froschhirns. Braunschweig, 1885. 6. STEINER, J. Die Functionen des Centralnervensystcms und ihre Phylogenese. II. Abtheilung, Die Fischc. Braunschweig, 7. GOLTZ, FR., and EVVALD, J. R. Dcr Hand mit verkurztem Riickenmark. Pfliigers Archiv, Bd. Ixiii. Bonn, 1896.

8. BICKEL. Beitrdge zur Ruckenmarksphysiologie des Aales. Pfliigers Archiv, Bd. Ixviii. 9. MEYER, ADOLF. Critical Review of the Data and General Methods and Deductions of Modern Neurology. Journal of Com- parative Neurology, vol. viii., 1898. It is apparent from the foregoing that in the central nervous system of Vertebrates only segmental gan- glia and only segmental reflexes appear. Superior centres, a " centre of coordination " for instance, can- not exist. Irritability and conductivity suffice to pro- duce coordination in Medusae, in the heart, in the respiration of Limulus, and in the movements of the earthworm and the salamander. Schrader has rightly expressed it : the nature of the nervous connections alone determines the cooperation of different seg- ments in a common activity. Some of these connec- tions require special mention, for instance, those in which decussation and semidecussation of fibres ap- pear. Possibly the most familiar example of semi- decussation is found in the optic nerves. Here the fibres cross, so that while each eye has its own special nerve each tract contains fibres from both eyes. The fibres of the temporal half of the retinae pass through the chiasma uncrossed (that is, remain on the same side of the head and brain), the fibres which come

from the nasal side of the retinae undergo a decussa- tion, i. e., they cross to the other side of the head and brain. The left optic tract contains fibres from the temporal side of the left eye, and from the nasal, or internal, side of the right eye. If the left tract be cut, the left sides of both retinae become blind, and the patient recognises nothing more in the right half of the field of vision. This is a case of hemianopia. A similar semidecussation also occurs in the motor nerves of the eye. For the time being we may con- sider the various muscles of each eye as a unit. In the lateral movements of our eyes the rectus externus of one eye and the rectus internus of the other co- operate. If we assume an inherited connection be- tween the retinal elements and the movements of the eyes, the right externus and the left internus must be innervated by the left half of the brain. The nerve- fibres of the externi must thus be crossed, those of the interni not crossed. The semidecussation in this case naturally occurs in the brain, and not peripherally. The pathological expression of this motor semidecus- sation is the deviation conjugte, which is a motor affection, corresponding to the sensory affection, hemianopia. We can only expect to find these semidecussations where symmetrical organs always receive equal innervations, as in the case of our eyes. Our arms and legs can move independently of each other, but in lower Vertebrates the case is different. The symmetrical fins of the fish receive equal innerv- ations. I have shown that associated changes of

position of the eyes and fins can be produced by de- struction of an ear or the acoustic nerve (i). If we destroy in a shark the left auditory nerve or the left side of the medulla, where the auditory nerve enters, the left eye of the animal looks down, the right up. This change of position of both eyes suggests that the relative tension between the muscles that raise the eyes has changed. In the left eye the tension of the lowering muscles predominates over that of their antagonists ; in the right eye the reverse is the case. The fins, likewise, show associated changes of posi- tion. The left fin is raised dorsally, the right is bent ventrally. While it can be said that both eyes are rolled about the longitudinal axis of the animal toward the left, the fins are rolled about the same axis to the right. Although the pectoral fins show the associated changes of position most clearly, these changes also exist in all the remaining fins, only with the difference that the amount of the change of position decreases the farther the segment is removed from the point of operation. The influence of the operation must de- crease as the distance of a ganglion increases. The resistance to the transmission of the change increases with the distance.

These observations enable us to draw a conclusion concerning the connection of the muscles with the right and left halves of the corresponding ganglia. We may assume that a permanent decrease, but not a permanent increase in the tension of the muscles can result from the destruction of one part of the brain. It is thus the muscles directly or indirectly connected with the left side of the medulla oblongata (in the acoustic segment) which show a decrease in tension after the destruction of the left ear. Accordingly, the left side of the medulla is connected with the raising muscles of the left eye and the lowering muscles of the right, eye, as well as with the lowering muscles of the left pectoral fin and the raising muscles of the right pectoral fin. If we start with the idea that all the muscles of an eye or a fin form a common whole, a kind of semidecussation is present. It is, however, not only the muscles of the fins that undergo such changes of tension, but probably also the muscles of the spinal column.

If the symmetrical muscles of the organs of loco- motion possess different tension, the usual stimuli for locomotion must naturally lead to unsymmetrical in- stead of symmetrical movements. When the lower- ing muscles predominate in the right pectoral fin and the raising muscles in the left, the animal, when these fins are used, will come under the influence of a couple of forces which must produce a rolling movement around the longitudinal axis of its body toward the left. As long as the animal swims slowly, rolling mo- tions do not occur, for they are compensated for. The friction of the fish in the water will suffice to destroy a slight rolling motion. But if the animal attempts to swim rapidly, e. g., if it be excited, it be- gins to roll. These rolling motions are called forced movements, a poorly selected term. The same move-

If a fish whose progressive movements are deter- mined by the sculling motions of the tail turns to the right, the tail moves with greater force toward the right than toward the left. This condition might be made permanent if it were possible to weaken the muscles on the left side of the spinal column. This occurs when the right side of the acoustic segment of the medulla is destroyed. The fish moves in a circle toward the right. We also obtain circus-motions to- ward the right if we destroy the ventral portion of the left optic lobe. Hence, fibres must pass from the ventral portion of the left optic lobe to the right acoustic segment of the medulla. After such an

operation, an increase in the tension of the skeletal muscles occasionally shows itself, for the fish may lie permanently bent into a circle without being able to straighten itself out again.1 Such a fish can no longer swim straight ahead. The difference in the tension of the muscles on the two sides of the animal is, how- ever, usually not so great, in which case the circus- motions will appear only spasmodically, for example, when the animal is excited.

One-sided division of the spinal cord and of the medulla behind the acoustic segment produces no forced movements (2 and 3). On the other hand, roll- 1 If such a fish be decapitated the curvature of the body remains. It may even remain after death. We have to deal with an organic change in the muscles, caused by the operation. ing and circus-motions may occur after injury to the brain in front of this segment, wherever places are met with which are directly or indirectly connected with the acoustic segment. This is the case, for in- stance, after a one-sided lesion of the pons or the cerebral hemispheres in rabbits and dogs. In both animals circus-motions occur after destruction of the cerebral hemispheres, in rabbits toward the intact side, in dogs toward the injured side. All the facts prove that the semidecussations take place in the vicinity of the acoustic segment and not farther down. In man, so far as I know, circus-motions have never been ob- served ; this is probably due to his upright walk. It would be interesting to make the experiment of having patients afflicted with certain diseases (for instance, diseases of the inner ear) walk on all fours (with closed eyes) and to observe whether circus-motions occur.

As we have already mentioned, it is a well-known fact that in Arthropods after destruction of one half of the supraoesophageal ganglion circus-motions can occur. That they need not occur has been shown by Miss Hyde and also by Bethe (4 and 5). According to the investigations of the latter author these circus- motions in Invertebrates are called forth by very dif- ferent disturbances in the muscle-tension. It is often due simply to a disturbance in the muscle-tension of the extremities of one side, the other side being ap- parently normal. In Crustaceans, associated changes of position of the extremities can also occasionally

occur after destruction of one half of the supracesoph- ageal ganglion. In Cephalopods von Uexkull has 36 shows the predominance of the flexors of the legs over the ex- tensors on the left side of the body of a Limulus. In this animal the right half of the brain had been de - stroyed and it showed circus-mo- tions toward the left. Steiner has made a peculiar applica- tion of the facts of forced movements. He imagines that the ability to move forward is a spe- cific "function" of the brain, and has believed that it would be possible by means of this criterion to decide whether or not a gan- glion of a lower animal should be called a brain. The

facts of comparative physiology do not favour this con- ception. Spontaneous progressive movements exist in Infusoria which possess no nervous system, and even in plant organisms, for example, in the swarmspores of algae. It is an important principle of physiological epistemology that a phenomenon which occurs gener- ally, cannot possibly be the specific function of an organ which is peculiar to a few forms only. Steiner soon found a fact that showed the erroneousness of his theory, the fact that the decapitated shark con- tinues to swim about in the tank. Schrader had like- wise found that the frog without a brain is still able to perform spontaneous progressive movements. Steiner maintains further that " the brain is defined by the general centre of movement in connection with the action of at least one of the higher sensory nerves." "In addition to its great simplicity this definition has still another advantage, namely, that it is satisfied by a single experiment ; because of the two elements of which the definition is made up, one element is always given anatomically. This is the higher sensory nerve, whose presence also vouches for its function. The one experiment that it is necessary to make has to prove that in addition to the sensory apparatus the gen- eral centre of movement also exists. The proof is then complete if the one-sided removal of the central nerv- ous part so changes the direction of the movements of the animals that a circus-motion, which is generally known by the name forced movement, takes the place of a forward movement" (Steiner). This idea is

likewise erroneous and easily leads to absurdity. One- sided destruction of the cerebral hemispheres in man produces no forced movements. Thus, according to Steiner, the cerebral hemispheres should not be- long to the brain. Second, according to Steiner, the ear must be a brain. One-sided lesion of the ear is sure to produce forced movements in a series of animals, and, moreover, the auditory nerve is a higher sensory nerve. I have mentioned this sub- ject at this place because it is a typical illustration of what plays on words in physiology lead to. It is not our task to find a definition for the word brain, but to gain an insight into the functions of the central nervous system. It is of minor importance what name we give to the different parts of the central nervous system.

In connection with this chapter we wish to call at- tention to the more recent experiments of Sherring- ton and H. E. Herincr from which it seems to follow that with the innervation of a muscle the relaxation of its antagonist results simultaneously. 2. LOEB, J. Ueber den Antheil des Hornerven an den nach Gehirnverletzung auftretenden Zwangsbewegungen, Zwangslagen und associirten Stellungsdnderungen der Bulbi und Extremitdten. Pflugers Archiv, Bd. 1., 1891.

3. STEINER. Die Functionen des Centralnervensystems und ihre Phylogenese. II. Die Fische. Braunschweig, 1888. 4. BET HE, A. Vergleichende Untersuchungen iiber die Func- tionen des Centralnervensystems der Arthropoden. Pfliigers Ai - chiv, Bd. Ixviii., 1897. 5. BETHE, A. Das Central nervensy stem von Carcinus manas. II. Mittheil. Arch. f. mikroskop. Anatomie, Bd. 1., 1897. 6. STEINER, J. Die Functionen des Centralnervensystems wir- belloser Thiere. Sitzungsberichte der Berliner Akademie der Wis- senschaften. 1890, I. S., 39.